1. Field of the Invention
Aspects of the present invention relate to an electrode for a fuel cell and a method of producing the same, and more particularly, to an electrode for a fuel cell, the electrode including a proton conductor which exhibits better proton conductivity at high temperatures of 100° C. or higher in a non-humidified condition than conventional proton conductors and can be manufactured at low temperature and a method of producing the electrode.
2. Description of the Related Art
Fuel cells can be classified into proton exchange membrane fuel cells (PEMFCs), phosphoric acid fuel cells (PAFCs), molten carbonate fuel cells (MCFCs), and solid oxide fuel cells (SOFCs) according to the type of electrolyte used in the cells. The operation temperature and materials of constitutional elements of fuel cells are changed according to the type of electrolyte.
Proton conductors can be used both in electrolyte membranes and in electrodes.
Electrolyte membranes act as separators preventing a physical contact between anodes and cathodes and as ion conductors transporting hydrogen ions (protons) from anodes to cathodes. Here, proton conductors, distributed in the electrolyte membranes, act as ion conductors.
Proton conductors are generally made of a perfluorosulfonated polymer called Nafion. Such perfluorosulfonated polymer-based proton conductors are excellent in mechanical strength, chemical stability, and ionic conductivity, but cannot be used at temperatures above 80° C. due to loss of water. Therefore, fuel cells using such perfluorosulfonated polymer-based proton conductors lack high-temperature operating capability.
In view of these problems, research into non-humidified polymer electrolytes is actively being carried out, based on mainly polybenzimidazole (PBI)-phosphoric acid (H3PO4) systems using phosphoric acid as a proton conductor.
However, the phosphoric acid used in the PBI-phosphoric acid systems is a fluid liquid, and thus, is not uniformly distributed on surfaces of catalyst/carbon particles constituting electrodes but locally soaked in spaces between the catalyst/carbon particles, causing non-uniformity problems.
That is, a redox reaction on electrodes occurs at a surface of a catalyst. At this time, the redox reaction most actively occurs at a portion of a catalyst in the vicinity of a liquid phosphoric acid where mass transfer from a vapor phase and mass transfer to a liquid phase smoothly occur. However, a portion of the catalyst surrounded by the liquid phosphoric acid where the redox reaction is active in the vicinity of the liquid phosphoric acid but where the mass transfer from the vapor phase inactively occurs is not subjected to the redox reaction. As a result, overall catalyst efficiency is reduced.
In addition, a phosphoric acid present in an electrolyte membrane or an electrode may cause the corrosion of a carbon bipolar plate due to its leakage. Here, the “corrosion” indicates formation of foreign substances through reaction between a leaked phosphoric acid and a functional group of a carbon surface. Such a corrosion reaction can be prevented by heat treating a carbon bipolar plate at 2,800° C. or more to remove the functional groups. In such a case, however, the manufacturing costs increase considerably.
In view of the above-described disadvantages of phosphoric acid, use of metal phosphate, such as tin phosphate (SnP2O7) or zirconium phosphate (ZrP2O7) as a proton conductor has been considered.
However, metal phosphate preparation involving high-temperature treatment above 500° C. cannot be performed in-situ with preparation of a platinum-carbon supported catalyst which is fragile at temperatures above 400° C.
Proton conductors manufactured according to conventional techniques are shown in FIGS. 2A, 2B, and 3. FIGS. 2A and 2B show proton conductors made of tin phosphate (SnP2O7) surrounded by phosphoric acid. Referring to FIGS. 2A and 2B, many proton conductor particles are agglomerated due to the use of the phosphoric acid. FIG. 3 shows a proton conductor made using 85% phosphoric acid (H3PO4) and boric acid. Referring to FIG. 3, BPO4 particles are surrounded by the phosphoric acid and considerably agglomerated. These conventional proton conductors are non-uniformly dispersed in a catalyst layer since they have a tendency to be easily agglomerated, and change from a solid state to a fluid state over time due to their moisture absorptivity, thus gradually blocking pores that are channels for mass transfer.